The goal of this project is to study the mechanisms of regulation of viral gene expression in murine cells infected by Moloney murine sarcoma virus (MSV 124) and Moloney murine leukemia virus (MLV cl. 1). The expression of sarcoma-like sequences in "normal" fibroblasts will also be studied. Clones of NRK cells producing MSV in vast excess (100-fold) over MLV, as well as clones producing equivalent amounts of both viruses, have been isolated. By using complementary DNA reagents that anneal specifically to defined regions of the viral chromosomes, the levels of virus specific intracellular RNA and the number of integrated proviral DNA copies will be determined. Pulse labelled virus-specific nuclear primary transcripts and cytoplasmic messenger RNA will be purified and characterized by specifically designed molecular hybridization techniques. The possible existence of a correlation between the specificity of proviral DNA integration sites and the levels of viral gene expression will be investigated. The regulation of the expression of endogenous sarcoma-like sequences which are not expressed under normal tissue culture conditions will be studied. The structure and sequence of the nuclear primary transcripts and cytoplasmic mRNA carrying the "sarcoma-like" cellular information will be analyzed by methods analogous to those used for the virus-specific sequences. It is hoped that the parallel investigation of the structure and mode of expression of the viral transforming gene(s) and of related "cellular" genes will lead to a better understanding of the molecular mechanism of viral oncogenesis.